Field
The present application relates to the determination of free volume in a rock sample.
Description of Related Art
In shale or coal, gas is adsorbed on the surface of the kerogen. This gas is referred to as “adsorbed gas.” Gas is also freely distributed in the pore space of the shale. This is referred to as “free gas.” The total gas in place (GIP) is the combination of adsorbed gas and free gas. Depending on the initial pressure of the reservoir, as free gas is produced and the pore pressure falls, adsorbed gas will be liberated, or desorb, from the surface of the kerogen.
Scientists sometimes use canister desorption tests to determine the total GIP from cores. Immediately upon retrieval, freshly cut core samples are sealed in canisters and sent to the laboratory for testing. The gas is removed from the canister, volumetrically measured and compositionally analyzed as a function of time. A plot of gas produced over time can be used to estimate the GIP for the core sample at reservoir conditions. This analysis is sensitive to the amount of time it takes to retrieve the core from downhole.
To determine adsorbed gas volume for shales engineers use pressure relationships that estimate the sorptive potential of the rock. Samples are pulverized to maximize surface area and then heated to drive off any adsorbed gas. Alternatively, the samples may be saturated with water or used in “as received” condition. Samples are then exposed to methane at increasingly high pressure while held at a constant temperature (i.e., isothermally). A sorption isotherm is a laboratory measurement performed on a representative sample to determine the gas storage capacity as a function of pressure. The term “isotherm” is used because the experiment is conducted at constant temperature, which is generally set to be equal to the temperature of the reservoir.
The volume of gas adsorbed by the rock sample, presented in units of standard cubic feet/ton (scf/ton), is described by a Langmuir isotherm curve such as that shown in FIG. 1. The Langmuir isotherm (labeled gas content in FIG. 1) is derived from crushed rock samples and quantifies a rock's adsorbed storage capacity. The Langmuir volume, VL, is the theoretical limit for gas adsorption at infinite pressure. Storage capacity at a given pressure, p, can be determined from the Langmuir isotherm. The Langmuir pressure, PL, is defined as the pressure at half the Langmuir volume.
As shown in FIG. 2, using the Langmuir isotherm, the total GIP for a specific reservoir can be determined as a function of pressure. The total gas is the gas adsorbed to kerogen plus free gas stored in pores. At low pressures, adsorption is an effective gas storage mechanism. As the pressure increases, pore gas correspondingly increases. Productivity of most of the organic shale reservoirs being developed today is driven by the volume of pore gas. Desorption becomes important as the bottomhole flowing pressure declines.
Once an isotherm is established, the storage capacity of the rock can be determined by referencing the pore pressure of the formation, which is representative of the in situ reservoir pressure. More and more isotherm tests are being performed on low adsorption shales. Although samples with total organic carbon (TOC) content of less than 2.5% are not typically run, running of low TOC content samples is becoming more common by some isotherm labs. However, existing isotherm testing on such low adsorption shales may generate isotherm data that indicates a negative slope at higher pressures instead of flattening out as shown in FIG. 1.